Young ignorant founder here. If a good email practice can be obtained by taking all those measures, but slack doesn't need the understanding, training, policies etc then surely slack is the better alternative. Just because it 'can be done' with email doesn't make it nonconditionally better.
So you'd rather give monkeys a golden hammer than train humans to use any tool effectively? Very interesting, young ignorant founder ..
Hint: its the people, not the technology, that make all the difference in the world. Invest in the people and it won't matter what technology they use.
If tools are evaluated by their leverage, and if Slack needs less effort (in form of training, etc) to achieve the same result as email/IRC, then isn't it reasonable to suggest that Slack is better? However, I think it is interesting to think about the method they used to achieve that increased leverage. In Slack's case it may not be an entirely new set of features, but just more optimal designs and implementations of existing feature sets. Should that necessarily "count for less"? I would argue no, if tools are evaluated by their leverage.
Every few years, generations of technologists evolve from their schooling, and come out into the workforce - competent, yes, but aware, no. Awareness of previous technologies is the problem: if you've never actually used e-mail productively, you may not be aware that people use it extremely productively, so you pitch an alternative - i.e. re-invent/NIH some 'replacement' technology that has a lot of fancy bells and whistles, but ends up re-implementing the stuff that your grandaddy programmers solved, decades ago.
Slack is an example of this - I see nothing in it that I can't do, effectively, with a well-run mailing list and well-configured mail applications, with company policies and, most of all TRAINING of employees/team-members on how to use it properly. The same goes for IRC. IRC and Email already solved the problems of Slack: its just that the mechanism (i.e. understanding) of applying those technologies is not evenly distributed.
It comes in waves, and I think it can be traced back to the timing of graduation schedules across academia.